In order to produce semiconductor components, in lithography by use of scanners or steppers the structures of masks, also referred to as reticles, are transferred to wafers. In order to ensure that the masks used are free of defects, they are subjected to extensive examinations before use in wafer exposure. The methods disclosed in this patent specification are also generally applicable to substrates which comprise structure elements.
One method for examining masks is the highly accurate measurement of the positions of structures on masks. This is referred to as “registration” or “photomask pattern placement.” Using a microscope, referred to as a position measuring device (registration tool), specific check structures—referred to as “registration pattern”—on the masks, such as, for example, squares, crosses or angles, are measured and compared with their setpoint positions. Positions of structures on the mask which are part of the used structures of the mask are also measured. This is referred to as “Real Pattern Registration.”
In order to examine a mask, the dimensions of structure features, such as the width of lines or the diameter of contact holes, are also measured. In one variant, the smallest structure features that can still be represented are measured. These are also referred to as critical dimensions, CD for short. Specific microscopes such as, for example, the WLCD from Zeiss are used for carrying out these measurements.
In the examinations mentioned, a corresponding microscope is used to image an aerial image of the structure of a mask onto a light-sensitive spatially resolving detector, such as a charge coupled device (CCD) chip, for example. Under the control of a computer, the aerial images imaged onto the detector are recorded and stored as files.
Besides aerial images, phase images of the masks can also be determined. It has been recognized, as described in DE10201.5218917 and US patent application publication 2017091921, that phase images have a higher edge steepness than intensity images. The entire contents of DE102015218917 and US2017091921 are incorporated by reference. The accuracy and reproducibility of the localization of structure features can thus be increased by the analysis of a phase image.
In order to evaluate the aerial images and the phase images, often the objective is to determine the distance of a structure feature in two images to be compared.
By way of example, in order to determine the position of a structure feature of a measured aerial image, the position thereof can be correlated with the structure feature of a simulated aerial image. The simulation of the aerial image is effected from the mask design underlying the mask to be examined.
In a further example, the aerial image or the phase image of a registration pattern is recorded by a position measuring device. A further aerial image is determined by mirroring of the aerial image. The mirroring is effected for example on two mirror planes, which are embodied parallel to the X axis and to the Y axis and which intersect at the midpoint of the aerial image. The distance between the two aerial images can be determined by the correlation of said aerial images. From said distance it is possible to determine the distance between the measured registration pattern and the setpoint position thereof. This method is explained in detail in the published patent application DE102010047051 and U.S. Pat. No. 8,717,581. The entire contents of DE102010047051 and U.S. Pat. No. 8,717,581 are incorporated by reference.